A conductive paste means a paste containing conductive fine particles in the solid component. Generally, for the conductive particles, metals such as gold, platinum, silver, and palladium which are not oxidized in the air are used. Conventionally, the conductive paste is largely classified into (1) high-temperature baking conductive paste which can realize a satisfactory conductivity by fusing silver particles, whereas the base material is limited to ceramics or the like, and (2) polymeric conductive paste which can be applied to a wide range from glass and epoxy printed boards to films, whereas the electrical resistance is relatively high so as to obtain conduction by means of contact between metal particles due to heat shrinkage at the time of curing a binder.
When such a polymeric conductive paste is used to form, for example, a conductive circuit on a flexible backing for a printed board, the polymeric conductive paste is used to form a conductive circuit pattern on a plastic film such as polyethylene terephthalate or polyimide by screen printing or the like, followed by heating and curing a binder in the formed pattern coating film, so as to improve the conductivity and the durability, and to ensure the adhesiveness onto the film.
In particular, the polymeric conductive paste using silver or a silver compound is used for forming various wirings in or between electronic parts, and electronic circuit patterns, since a stable conductivity is readily realized and the heat conduction characteristic is also satisfactory.
In a step for printing conductive circuit patterns, involved in the miniaturization of circuits, the printing precision is required to be increased as high as possible, while the precision is limited by the average particle diameter of the conductive particles. Therefore, in order to obtain a satisfactory printing precision, the conductive particles in the conductive paste are required to be in a state where they are dispersed into primary particles, and further the primary particles are completely coated with a resin or the like, that is, a highly dispersed state.
If the coating of the resin or the like around the highly dispersed primary particles is insufficient, the activity of the particles is increased as the particle diameter gets smaller. Therefore, the particles in the conductive paste are aggregated, the viscosity easily rises over time, and in an extreme case, gelation occurs.
In this manner, in order to precisely and stably form fine wiring patterns, the conductive particles are required to be stably dispersed into primary particles, and moreover each particle is required to be coated with a resin. On the other hand, if the resin coating around the conductive particles is too thick, resin components remaining between the particles interfere with the electrical conduction between particles, and thus a satisfactory conductivity is prone to be unobtainable. That proneness becomes more remarkable as the particle diameter of the conductive particles to be used gets smaller.
Consequently, the amount of resin used for dispersion and coating over the surface of the conductive particles is preferably necessarily a minimum. Preferably, the dispersibility of conductive particles, the adhesiveness of conductive paste to the base material, and the film forming property thereof become satisfactory with less amount of resin.
Conventionally, in production of a paste by dispersing conductive particles in a resin, in order to improve the dispersibility, various dispersants are used, which include nonionic dispersants such as ethylene oxide or propylene oxide-added ester compounds of higher fatty acids, sorbitan fatty acid ester compounds, ethylene oxide or propylene oxide-added ether compounds of polyhydric alcohols such as sorbitan, or ethylene oxide or propylene oxide-adducts of alkylbenzenes; anionic dispersants such as alkali salts of alkylbenzene sulfonic acids, alkali salts of higher alcohol sulfate esters, phosphate ester compounds, higher fatty acids, or ethylene oxide or propylene oxide-added sulfate alkali salts of higher fatty acids; and cationic dispersants such as quaternary ammonium salt type.
However, even if these dispersants are used, in a conventional method, for example, a method of dispersing conductive particles in a resin using a disperser or a kneader, the dispersibility of the conductive particles has not been able to be sufficiently improved, and the conductivity of conductive paste has not been able to be kept satisfactory.
In particular, in the case of a silver paste using silver particles as conductive particles, the true specific gravity of the silver particles is 10.5, and the silver particles are easily sedimented with a high reaggregability. Therefore, there is a problem in that time and effort are required for a redispersing step for use after a long term preservation. Consequently, long term preservation stability and satisfactory redispersibility have been demanded.
In response to these problems, an attempt has been made to obtain satisfactory dispersibility and stability over time by using an organic vehicle and an anionic surfactant formed of a sulfosuccinate containing an alkyl group (see Patent Document 1). However, mere usage of this method described in Patent Document 1 is not enough to improve the redispersibility of the sedimented particles. Furthermore, when the line width of the conductive circuit is narrow, it is not enough to precisely print the wiring pattern.
Moreover, the amount of resin used for dispersion is not reduced to the necessarily minimum, and thus it is not enough in terms of the conductivity, either.
On the other hand, in production of a dispersion liquid using metal particles, a vacuum freeze drying method is used. For example, in production of a tantalum powder coating material for producing anode elements for electrolytic capacitors, a method is known in which a tantalum powder and a dispersant are mixed in a solvent, and vacuum freeze drying is performed to adsorb the dispersant into the surface of the tantalum particles (see Patent Document 2).
However, even when being similarly to a metal dispersion liquid, there is no example in which vacuum freeze drying is applied to the production of a conductive paste having greatly different usage and greatly different required characteristics, and the relationship between the dispersibility and the conductivity has not been suggested. Still more, selection of a dispersant for realizing a satisfactory conductivity is not disclosed at all.
As above, in order to sufficiently bring out the characteristics of a conventional polymeric conductive paste, application of an optimum dispersion method and selection of a dispersant suitable for the dispersion method are important, and the dispersant for realizing a satisfactory conductivity of the conductive paste and the dispersion method thereof have been demanded.
In particular, recently, in order to reduce the electrical resistance of a conductive circuit formed from a conductive paste, it is discussed to use a silver paste capable of low-temperature baking by highly miniaturizing the diameter of silver particles, or using a particulate silver compound containing a particulate silver oxide or a tertiary fatty acid silver salt. In this low-temperature baking silver paste, in the case of the finely particulated silver particles, adjacent particles are fused by heating at 300° C. or less so as to reduce the electrical resistance of the conductive circuit. Moreover, in the case of the particulate silver compound, the silver compound is reduced to become a silver-metal, by heating at 300° C. or less or heating under the presence of a reductant, and adjacent silver particles are fused so as to reduce the electrical resistance of the conductive circuit (for example, see Patent Document 3).
Conventionally, a polymeric conductive paste is obtained by adding a binder formed from an acrylic resin, an epoxy resin, a polyurethane resin, a polyester resin, or the like, an organic solvent, a curing agent, a catalyst, and the like, and dispersing and mixing globular or flaky conductive particles therein. The conductivity is obtained by a contact between the conductive particles due to curing shrinkage when the binder is cured. Therefore, the electrical resistance becomes relatively high, and the cohesive power of the cured resin is changed due to the temperature change or the like, thus accompanied by a drawback in that the electrical resistance of a conductive circuit, which is formed by using such a polymeric conductive paste or the like, is easily changed.
The low-temperature baking silver paste compensates for such a drawback. By using this silver paste, a conductive circuit having a satisfactory conductivity can be formed on a plastic film such as PET.
However, such silver particles which are microparticulated into submicrons or less, or particulate silver compound to be used for a low-temperature baking silver paste, have a high reactivity, and are both difficult to handle in a dried powder state. In particular, the particulate silver compound is very high in the speed of reduction reaction, and thus has to be preserved in a solution such as water or a low-reducibility solvent. In particular, if an alcohol solvent or the like is used for the dispersion solvent, reduction progresses during the dispersion of the silver paste, and instead of dispersion, fusion might occur between the silver particles. Therefore, it is further demanded to disperse these silver or silver compound particles into primary particles, and to coat the surface of each particle for stabilization.
Furthermore, in order to obtain an inherent high electrical conductivity in such silver paste, the silver particles in the silver paste have to be satisfactorily dispersed while keeping a high silver content. In addition, the stabilization has to be performed by a necessarily minimum amount of resin, to thin the coating film thickness on the surface of the particle as much as possible, so that the adjacent particles are readily fused by low-temperature baking. If the coating film formed on the surface of the silver or silver compound particles is too thick, it is difficult for the adjacent particles to be fused, and the packing density of the silver or silver compound is decreased, thus the inherent merit of such low-temperature baking silver pastes can not be obtained, which provides a satisfactory conductivity even in low-temperature baking.
Therefore, with respect to these low-temperature baking silver pastes, selection of a dispersant which concurrently satisfies the stabilization by means of coating individual silver or silver compound particles, the improvement of dispersibility, and the improvement of conductivity by means of low-temperature baking, and a method for producing thereof are even more important.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2000-231828
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-006502
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2003-309337